This invention relates to apparatus for distinguishing between speech and certain other signals.
There are several situations in the communications field where it would be advantageous if a speech signal could be distinguished from other signals in an automatic and reasonably reliable manner. For example, in addition to speech and noise the HF communications frequency band accommodates Morse-coded messages and also data traffic which may take several different forms, and it is highly desirable in, for example, a communications centre or monitoring station where the only transmissions of interest are speech that the audio output of a communications receiver be automatically disabled when no speech is actually being received and be automatically enabled otherwise, thereby preventing data traffic, Morse-coded messages, noise etc. from reaching the ear of an operator but allowing speech through. This is particularly important where the operator has to listen to several radios at once. The provision of such a facility could also result in a reduction in the average power consumption of the receiver, which would be of obvious importance if the receiver were battery-powered. Another potential use of such a facility is in an automatic rebroadcast station where, ideally, the transmitter should be switched on only when a wanted (speech) signal is received.
The automatic disabling of the audio channel of a receiver in the absence of a wanted signal is often referred to as "squelch" and it is common practice, for example, to provide such a facility in VHF FM broadcast receivers to mute the audio channel in the absence of a received carrier of a predetermined amplitude, thereby inter alia preventing noise from reaching a listener when the receiver is tuned from one broadcast programme to another. Threshold-responsive carrier-operated squelch is very reliable in an FM system but, obviously, such a facility cannot distinguish between the different modulations which may be encountered, for example between speech and data. It is therefore only of limited assistance in environments such as the communication centre or monitoring station referred to above.